Portable chair generally consists of a collapsible chair frame constructed by metal tubes, and a chair seat made of durable fabric.
FIG. 1 illustrates a type of portable chair which includes a chair frame A1 which is constructed by a set of interlocking tubes A11 to pvoide a back frame section A2 and a seat frame section A3 for supporting a fabric seat B1. The interlocking tubes A11 of the seat frame A1 are often arranged in inclined manner so that the chair frame A1 can be easily opened to proivde a rigid cross-support for use and be folded up for storage.
The conventional method of attaching the fabric seat B1 to the chair frame A1 is to affix an upper end B2 of the fabric seat B1 to a top end of the back frame section A2 and to have two tube placement holes B11 provided at two front corners of the fabric seat B1 for mounting a front end B3 of the fabric seat B1 on two the seat frame. Each of the placement holes The conventional fabric seat is attached to the seat frame by slipping the tube placement holes over the inclined frame tubes of the seat frame portion of the chair. The seat can then be hooked over the top of the back frame to secure the back of the fabric seat to the frame. This means that at least two tube placement holes on the front portion of the fabric seat for attaching the fabric seat to the front portion of a seat frame are needed for the conventional fabric seat in order to attach the fabric seat to a seat frame.
All typical frame tubes A11, including the two front frame tubes A11a, are cylindrical hollow tube and the tube placement holes B11 of the conventional fabric seat B1 are cut in circular shape and to the size of the frame tube A11 as if the fabric seat B1 were resting perpendicular to the frame tube A11. This allows the tube placement hole B11 of the fabric seat B1 to slide onto the inclined frame tube A11 easily and snugly. This conventional design, however, has inherent problems when the chair is used. Since the two front frame tube A11a are supported inclinedly and pivotally in cross manner but not in vertical manner, an individual sitting down on the fabric seat B1 causes stress to the lining of the tube placement hole B11 of the fabric seat B1. This causes the tube placement hole B11 to be stretched and misshapen, or worse, torn open, as a pulling pressure is applied to the lining of the tube placement holes B11 as an individual sits on the fabric seat B1.
Conventionally, in order to reinforce the structural integrity of the tube placement hole B11, a circular support ring made of a rigid material, such as metal or hard plastic, can be mounted on the placed within the tube placement hole.
However, since the support ring lining of the tube placement hole B11 is also cylindrical and just wider then the width of frame tube, it merely shifts the point of stress on the fabric seat B1. The circular support ring merely sits perpendicularly to an axis of the frame tube B11. In this situation, if an individual sits down, an angle is created on the fabric seat B1 from the downward pulling force on the fabric seat B1 at the point where the edge of the support ring ends. A person's weight causes the fabric seat B1 to be pulled away from the edge of the support ring. Therefore, the fabric seat B1 at the edge of the support ring is pulled from a stress created at the edge of the support ring. At the same time, the ring itself causes an opposite downward pulling force on the fabric seat B1 at the same stress point. As a result, two things occur. First, the fabric seat B1 will be distorted at the edge of the support ring, creating a pointed ridge at the edge of the support ring. Second, a stress point is created at edge of the support ring where the angle is created.
In other words, the stress on the fabric seat B1 has simply been transferred from the lining of the tube placement hole B11 to a point just off the tube placement hole B11. The stress created will then cause a tear, not at the tube placement hole B11, but along the edge of the support ring. Although additional lining layers B12 may be added to the corners of the fabric seat B1 around the tube placement hole B11 to delay the tearing effect, ultimately the stress will cause the fabric seat B1 to tear. Also, the fabric will be permanently misshapen at the stress point over a period of continued use.